This works for me, there's probably better jquery filters I could use.

openerp.kkproject = function (instance) {
  instance.mail.ThreadMessage.include({
        start: function () {
            this._super.apply(this, arguments);
            this.$("div.oe_msg_body_long, div.oe_msg_body_short").children().each(function() {
                if ($(this).width() > $("div.oe_msg_center").width() ) {
                    $(this).wrap('<div style="overflow-x:scroll"></div>' );
                }
            });
        },
        on_expand: function (event) {
            this._super.apply(this, arguments); 
            this.$("div.oe_msg_body_long, div.oe_msg_body_short").children().each(function() {
                if ($(this).width() > $("div.oe_msg_center").width() ) {
                    $(this).wrap('<div style="overflow-x:scroll"></div>' );
                }
            });
        }
    });
};